This invention relates to a non-aqueous electrolytic solution and a secondary battery containing the same.
As an active material for a positive electrode which provides a practically usable lithium secondary battery, lithium transition metal oxides are believed to have a bright prospect. Of the lithium transition metal oxides, lithium cobalt oxide, lithium nickel oxide and lithium manganese oxide are known to exhibit high-performance battery properties. Therefore, research and development have been vigorously conducted mainly on these compounds to put such batteries into practical use. However, even in case of using these materials, various problems have to be overcome for making the batteries to reach a practically usable level.
One of the problems to be solved first is a problem of deterioration of battery properties in a high temperature environment. Deterioration of properties of lithium secondary batteries in a high temperature environment is caused by various factors. As such factors, there are illustrated, for example, change in properties of lithium transition metal oxide, decomposition of the electrolytic solution, and breakage of a film formed on a negative electrode.
In particular, lithium manganese oxides such as LiMn2O4 are inferior to lithium cobalt oxides or lithium nickel oxides in battery properties in the high temperature environment. Therefore, in cases when the lithium manganese oxides are used as an active material for a positive electrode, it is particularly required to solve the problem of deterioration of battery properties in the high temperature environment.
Thus, it has been attempted to improve battery properties of the lithium manganese oxides in the high temperature environment by replacing part of manganese atoms therein by other element. For example, J. Electrochem. soc., Vol. 145, No. 8 (1998) 2726 to 2732 discloses lithium manganese oxides in which part of manganese atoms are replaced by other elements such as Ga or Cr.
However, secondary batteries containing a non-aqueous electrolytic solution have been required to have an increasingly higher battery properties and, therefore, there have been strong demands for improvement of battery properties when used in the high temperature environment.
With the above-described circumstances in mind, the inventors have investigated in detail the cause of deterioration of battery properties in the high temperature environment. As a result, the inventors have surmised the mechanism of deterioration of battery properties in the high temperature environment as follows. That is, an acid generated by decomposition of the lithium salt used as an electrolyte supposedly accelerates decomposition of the active material for a positive electrode in the high temperature environment. For example, deterioration of battery properties in the high temperature environment in the case of using a lithium manganese oxide as an active material for a positive electrode and a fluorine-containing compound as a lithium salt is supposedly caused by dissolution of manganese into the non-aqueous electrolytic solution due to reaction between hydrofluoric acid having been generated by the reaction between the lithium salt and water and the lithium manganese oxide at a high temperature.
On the above-described supposition, the inventors have intensively investigated the problem of the deterioration of battery properties in the high temperature environment and, as a result, have found that the problem of deterioration of the battery properties in the high temperature environment can be solved in a different manner from the conventional manner of improving active materials for a positive electrode. That is, the inventors have found that incorporation of a compound which can trap an acid to be generated by decomposition of the lithium salt used as an electrolyte serves to depress decomposition of the active material for a positive electrode and that use of particular substituted pyridine compounds as compounds having a strong ability of trapping the acid (i.e., strongly basic compounds) can improve the battery properties in the high temperature environment, thus having completed the invention based on the findings.
That is, a first gist of the invention lies in a non-aqueous electrolytic solution comprising an organic solvent and a lithium salt, which further contains a pyridine compound represented by the following formula (1): 
wherein R1 to R5 each independently represents a hydrogen atom or a substituent composed of an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, an aryloxy group having 6 to 10 carbon atoms, a dialkylamino group having 2 to 8 carbon atoms, a 3-thienyl group, a cyano group, a fluoro group, an alkoxycarbonyl group having 1 to 6 carbon atoms, an arylcarbonyl group having 6 to 10 carbon atoms, an alkylcarbonyl group having 1 to 12 carbon atoms, a cyanoalkyl group having 1 to 4 carbon atoms, an alkoxycarbonylalkyl group having 3 to 13 carbon atoms, a pyrrol-1-ylmethyl group, a 1-pyrrolidinyl group, a 1-piperidino group, a phenyl group (provided that, in this case, two or more of R1 to R5 represent phenyl groups), a 1H-pyrrol-1-yl group, an alkoxyalkyl group having 2 to 12 carbon atoms, a dialkylaminoalkyl group having 3 to 18 carbon atoms, an aryloxy group having 6 to 10 carbon atoms, an arylalkyl group the aryl moiety of which has 6 to 10 carbon atoms and the alkyl moiety of which has 2 to 6 carbon atoms, an isothiocyano group, a dialkylaminocarbonyl group having 2 to 8 carbon atoms, a 5-oxazole group, a trifluoromethyl group, a 1-pyrrolidine-2,5-dione group, a 1H-pyrrol-1-ylalkyl group having 1 to 6 carbon atoms, a 4,5-dihydro-oxazol-2-yl group, a 1,3,4-oxadiazol-2-yl group, a nitro group, a 1-piperidinyl group, a 1-alkylpyrrol-2-yl group having 1 to 6 carbon atoms, a 4-1,2,3-thiadiazole group, a 2-1,3,4-oxadiazole group, a morpholino group and a 1-pyrrolin-2-yl group, with the proviso that, at least one of R1 to R5 represents aforesaid substituent and that, when R1 to R5 are a hydrogen atom or an alkyl group, at least one of R1 to R5 is an alkyl group having 2 or more carbon atoms and sum of the carbon atoms of R1 to R5 is 3 or more.
The second gist of the invention lies in a secondary battery, which comprises the non-aqueous electrolytic solution, a positive electrode and a negative electrode.
That is, the gist of the invention resides in the following aspects.
(1) A non-aqueous electrolytic solution comprising an organic solvent and a lithium salt, which further contains a pyridine compound represented by the following formula (1): 
wherein R1 to R5 each independently represents a hydrogen atom or a substituent composed of an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, an aryloxy group having 6 to 10 carbon atoms, a dialkylamino group having 2 to 8 carbon atoms, a 3-thienyl group, a cyano group, a fluoro group, an alkoxycarbonyl group having 1 to 6 carbon atoms, an arylcarbonyl group having 6 to 10 carbon atoms, an alkylcarbonyl group having 1 to 12 carbon atoms, a cyanoalkyl group having 1 to 4 carbon atoms, an alkoxycarbonylalkyl group having 3 to 13 carbon atoms, a pyrrol-1-ylmethyl group, a 1-pyrrolidinyl group, a 1-piperidino group, a phenyl group (provided that, in this case, two or more of R1 to R5 represent phenyl groups), a 1H-pyrrol-1-yl group, an alkoxyalkyl group having 2 to 12 carbon atoms, a dialkylaminoalkyl group having 3 to 18 carbon atoms, an aryloxy group having 6 to 10 carbon atoms, an arylalkyl group the aryl moiety of which has 6 to 10 carbon atoms and the alkyl moiety of which has 2 to 6 carbon atoms, an isothiocyano group, a dialkylaminocarbonyl group having 2 to 8 carbon atoms, a 5-oxazole group, a trifluoromethyl group, a 1-pyrrolidine-2,5-dione group, a 1H-pyrrol-1-ylalkyl group having 1 to 6 carbon atoms, a 4,5-dihydro-oxazol-2-yl group, a 1,3,4-oxadiazol-2-yl group, 2-yl group, a nitro group, a 1-piperidinyl group, a 1-alkylpyrrol-2-yl group having 1 to 6 carbon atoms, a 4-1,2,3-thiadiazole group, a 2-1,3,4-oxadiazole group, a morpholino group and a 1-pyrrolin-2-yl group, with the proviso that, at least one of R1 to R5 represents aforesaid substituent and that, when R1 to R5 are a hydrogen atom or an alkyl group, at least one of R1 to R5 is an alkyl group having 2 or more carbon atoms and sum of the carbon atoms of R1 to R5 is 3 or more;
(2) The non-aqueous electrolytic solution as described in (1), wherein R1 to R5 in the formula (1) each independently represents a hydrogen atom or a substituent composed of an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 3 to 20 carbon atoms, an alkoxy group having 1 to 4 carbon atoms, an aryloxy group having 6 to 10 carbon atoms and a dialkylamino group having 2 to 8 carbon atoms;
(3) The non-aqueous electrolytic solution as described in (1), wherein at least one of R1 to R5 in the formula (1) represents an alkyl group having 1 to 20 carbon atoms with the proviso that, when R1 to R5 are a hydrogen atom or an alkyl group, at least one of R1 to R5 is an alkyl group having 2 or more carbon atoms and sum of the carbon atoms of R1 to R5 is 3 or more;
(4) The non-aqueous electrolytic solution as described in any one of (1) to (3), wherein, when R1 to R5 in the formula (1) are a hydrogen atom or an alkyl group, at least one of R1 to R5 is an alkyl group having 3 or more carbon atoms;
(5) The non-aqueous electrolytic solution as described in any one of (1) to (4), wherein, when R1 to R5 in the formula (1) are a hydrogen atom or an alkyl group, sum of the carbon atoms of R1 to R5 is 4 or more;
(6) The non-aqueous electrolytic solution as described in any one of (1) to (5), wherein, when R1 to R5 in the formula (1) are a hydrogen atom or an alkyl group, sum of the carbon atoms of R1 to R5 is 60 or less;
(7) The non-aqueous electrolytic solution as described in any one of (1) to (6), wherein R1 and R5 in the aforesaid formula (1) are the aforesaid substituents;
(8) The non-aqueous electrolytic solution as described in (7), wherein R3 is also the aforesaid substituent.
(9) The non-aqueous electrolytic solution as described in any one of (1) to (8), wherein the pyridine compound is at least one member selected from the group consisting of 2-propylpyridine, 3-propylpyridine, 4-propylpyridine, 2-isopropylpyridine, 4-isopropylpyridine, 3-butylpyridine, 4-butylpyridine, 4-isobutylpyridine, 2-methyl-5-butylpyridine, 2-tert-butylpyridine, 4-tert-butylpyridine, 2,6-di-tert-butylpyridine, 2,6-di-tert-butyl-4-methylpyridine, 2,4,6-tri-tert-butylpyridine, 2-tert-butyl-6-methyl-pyridine, 2-tert-butyl-4-methylpyridine, 4-tert-butyl-2-methylpyridine, 2-tert-butyl-6-isopropylpyridine, 5-nonyl)pyridine, 2-pentylpyridine, 2-(3-pentyl)pyridine, 4-(3-pentyl)pyridine, 2-hexylpyridine, 4-octylpyridine, 2-undecylpyridine, 2-(1-butylpentyl)pyridine, 4-(1-propenylbutenyl)pyridine, 4-(1-butenylpentenyl)pyridine, 2,6-di-tert-butyl-4-(dimethylamino)pyridine, 2-(3-thienyl)pyridine, 2-cyanopyridine, 2-fluoropyridine, pentafluoropyridine, 2-dimethylaminopyridine, 2-methoxypyridine, 2-pyridinecarboxylic acid ethyl ester, 2-benzoylpyridine, 2-acetylpyridine, 2-(cyanomethyl)pyridine, 4-(3-phenylpropyl)pyridine, 2-pyridylacetic acid methyl ester, 3-(pyrrol-1-ylmethyl)pyridine, 4-(1-pyrrolidinyl)pyridine, 4-piperidinopyridine, 2,4,6-triphenylpyridine, 2-(1H-pyrrol-1-yl)pyridine, 2-methoxyethylpyridine, 4-(2-diethylaminoethyl)pyridine, 2-phenoxypyridine, 3-pyridylisothiocyanate, N,N-dimethylnicotinamide, 5-(pyrid-4-yl)oxazole, 3-trifluoromethylpyridine, 1-(3-pyridyl)pyrrolidine-2,5-dione, 4-(1H-pyrrol-1-ylmethyl)pyridine, 3-(4,5-dihydro-oxazol-2-yl)pyridine, 4-(1,3,4)oxadiazol-2-ylpyridine, 3-nitropyridine, 2,6-di(1-piperidinyl)pyridine 3-(1-methylpyrrol-2-yl)pyridine, 3-methoxypyridine, 4-(4-pyridyl)-1,2,3-thiadiazole, 2-(3-pyridyl)-1,3,4-oxadiazole, 2,6-dimorpholinopyridine and 2-(1-pyrrolin-2-yl)pyridine;
(10) The non-aqueous electrolytic solution as described in any one of (1) to (9), wherein the pyridine compound is a pyridine compound having a bonding energy of 16 kcal/mol or more with hydrofluoric acid determined according to the following calculation method:
(Method for Calculating Bonding Energy)
A bonding energy between the aforesaid pyridine compound and hydrofluoric acid is calculated according to ab initio method (program: Gaussian 94; base set: 3-21G); and the term xe2x80x9cbonding energyxe2x80x9d as used herein means a value obtained by summing the energy values of the pyridine compound and hydrofluoric acid determined by geometry optimization of each of them, and subtracting from the sum the energy value determined by geometry optimization of an adduct of the pyridine compound and hydrofluoric acid connecting to each other through nitrogen atom of the pyridine compound and hydrogen atom of hydrofluoric acid, that is,
(Bonding energy)=(Energy value of the pyridine compound) +(Energy value of hydrofluoric acid)xe2x88x92(Energy value of the adduct between the pyridine compound and hydrofluoric acid);
(11) The non-aqueous electrolytic solution as described in any one of (1) to (10), wherein the pyridine compound is contained in an amount of 0.001% by weight based on the sum of the organic solvent and the lithium salt to saturation;
(12) The non-aqueous electrolytic solution as described in any one of (1) to (11), wherein the lithium salt is a compound containing a fluorine atom or fluorine atoms;
(13) A secondary battery, which comprises the non-aqueous electrolytic solution described in any one of (1) to (12), a positive electrode and a negative electrode;
(14) The secondary battery as described in (13), wherein the positive electrode comprises an active material for a positive electrode, the active material for a possitive electrode being a lithium transition metal oxide;
(15) The secondary battery as described in (14), wherein the lithium transition metal oxide is lithium manganese oxide or lithium cobalt oxide;
(16) The secondary battery as described in (15), wherein the lithium manganese oxide is spinel type lithium manganese oxide;
(17) The secondary battery as described in (15) or (16), wherein the lithium manganese oxide is lithium manganese oxide wherein part of manganese sites are occupied by other element;
(18) The secondary battery as described in (17), wherein the other element occupying the manganese sites is at least one metal element selected from the group consisting of Al, Ti, V, Cr, Fe, Co, Li, Ni, Cu, Zn, Mg, Ga and Zr;
(19) The secondary battery as described in any one of (13) to (18), wherein the negative electrode comprises an active material for a negative electrode, the active material for a negative electrode being a carbonaceous substance; and
(20) The secondary battery as described in (19), wherein the carbonaceous substance is graphite having a d value of lattice plane (002 plane) in X ray diffraction of 0.335 to 0.340 nm.